Physostigma venenosum

Physostigma venenosum, the Calabar bean or ordeal bean, is a leguminous plant, Endemic to tropical Africa, with a seed poisonous to humans. It derives the first part of its scientific name from a curious beak-like appendage at the end of the stigma, in the centre of the flower; this appendage, though solid, was supposed to be hollow (hence the name from φῦσα, a bladder, and stigma).

Growth
The plant is a large, herbaceous, climbing perennial, with the stem woody at the base, up to 2 in in diameter; it has a habit like the scarlet runner, and attains a height of about 50 ft. The flowers, appearing in axillary peduncles, are large, about 1 in long, grouped in pendulous, fascicled racemes pale-pink or purplish, and heavily veined. The seed pods, which contain two or three seeds or beans, are 6 - 7 in in length; and the beans are about the size of an ordinary horse bean but less flattened, with a deep chocolate-brown color.

Toxicology
Calabar bean contains physostigmine, a reversible cholinesterase inhibitor alkaloid. The alkaloid physostigmine acts in effect like nerve gas, influencing communication between the nerves and muscles, and resulting in copious salivation, seizures, loss of control over the bladder and bowels, and eventually loss of control over the respiratory system, causing death by asphyxiation.

The main antidote to Calabar bean poisoning is the slightly less toxic tropane alkaloid atropine, which may often succeed; and the other measures are those usually employed to stimulate the circulation and respiration. Unfortunately, the antagonism between physostigmine and atropine is not perfect, and Sir Thomas Richard Fraser demonstrated that sometimes the action of the two drugs is summated and death results sooner than from either alone: namely, atropine will save life if three and a half times the fatal dose of physostigmine has been taken, but will hasten the end if four or more times the fatal dose has been ingested.

Historical uses
The people of Old Calabar used Calabar beans or 'E-ser-e' as an ordeal poison, and administered them to persons accused of witchcraft or other crimes. It was considered to affect only the guilty; if a person accused of a crime ingested the beans without dying, they were considered innocent. A form of dueling with the seeds was also practiced, in which the two opponents divide a bean, each eating one half; that quantity has been known to kill both adversaries. Although thus highly poisonous, the bean has nothing in external aspect, taste or smell to distinguish it from any harmless leguminous seed, and disastrous effects have resulted from its being incautiously left in the way of children. The beans were first introduced into Britain in the year 1840; but the plant was not accurately described until 1861, and its physiological effects were investigated in 1863 by Sir Thomas Richard Fraser. The bean usually contains a little more than 1% of alkaloids. Two of these have been identified, one called calabarine with atropine-like effects, and the other, the drug physostigmine (with the opposite effect), used in the treatment of glaucoma, anticholinergic syndrome, myasthenia gravis and delayed gastric emptying.